The present invention is directed, in general, to an apparatus and a method of determining the distance between a relief well, or borehole, being drilled and an existing cased target well. More particularly, the invention is directed to a method for directly obtaining the distance between a relief borehole and a target well through the measurement of alternating or static magnetic field gradients, or both, at the location of the relief well. The invention further provides a method for determining the direction to the target well from the relief well.
Although electronic methods for guiding the drilling of a relief borehole to intersect an existing, or target, well having a casing have been extremely successful, it has recently become apparent that often some additional way to guide the final portion of the relief borehole after it is within a few feet of the target is needed. This need arises particularly when the total depth of the target well closely coincides with the desired intersection point, which often occurs, or if complications arise with mathematical modeling in a situation where modeling is required for determination of the current flow on the target. During the drilling of a relief well, particularly after it is within a few feet of the target, static Earth field perturbations can be very large due to the ferromagnetic nature of the casing of the target well. The magnetic poles which generate these perturbations are "smeared out" along the length of the target well to such an extent that the present methods of mathematical analysis of the associated field perturbations have been virtually useless, and this problem has led to documented cases of drastic error in directing the relief well.